Biological implants, such as joint and dental prostheses, usually must be permanently affixed or anchored within bone. In some instances it is acceptable to use a bone cement to affix the prosthesis within bone. In the case of many joint prostheses, however, it is now more common to affix the joint prosthesis by encouraging natural bone growth in and around the prosthesis. Bone-to-implant interfaces that result from natural bone ingrowth tend to be stronger over time and more permanent than are bone cement-prosthesis bonds.
Optimal bone ingrowth requires that natural bone grow into and around the prosthesis to be implanted. Bone ingrowth and bone or dental prosthesis fixation can be enhanced by providing irregular beaded or porous surfaces on the implant. Although various materials, including titanium alloys, are biocompatible, they are not necessarily bioactive because they can neither conduct bone formation nor form chemical bonds with bone. Thus, enhanced fixation of implants within bone can be attained by coating the implant with a bioactive mineralized and/or ceramic material. Such coatings have been shown to encourage more rapid bone ingrowth in and around the prosthesis.
Calcium phosphate ceramics, especially hydroxyapatite, have been shown to conduct bone formation. Hydroxyapatite ceramic has been successfully applied as a coating on cementless metallic implants to achieve quick and strong fixation. Thermal plasma spraying is one of the more common methods used to produce hydroxyapatite coatings. However, the resulting plasma-sprayed hydroxyapatite coating is of relatively low density and is not uniform in structure or composition. The adhesion between the coating and substrate is generally not very strong, especially after long term exposure within the body. The generation of hard ceramic particles, resulting from the degradation of thermal plasma sprayed coating, and coating delamination, are major concerns. Also, implants or other polymers with porous structures or complex surfaces are difficult to coat uniformly using line-of-sight temperature plasma spraying.
A general overview of orthopedic implantable materials is given in Damien, Christopher J., and Parsons, Russell J., 1991, Journal of Applied Biomaterials, v2, 187-208. Information related to attempts to address these problems can be found, e.g., in U.S. Pat. Nos. 6,139,585; 6,051,272; 5,609,633; and in some of the publications disclosed herein. Each of these references suffers from one or more of the following disadvantages: weakness, brittleness or unevenness of coatings, a lack of chondrogenic or osteogenic activities, inability to promote the regeneration of periodontal tissues, and the problem of contamination with components which may cause severe immunological reactions.
Thus, a need exists for the production of improved enamel inspired materials which are appropriate for biomedical and dental applications and which overcome the problems discussed above.